A conventional bobbin holder is a device in which a rolling contact bearing structure disposed axially of the bobbin holder cooperates with the bearing itself to have a rotating-resistance value of substantially 0, and a "braking mechanism" in which a brake shoe 7Bd (see FIG. 7) presses against a rotating part by means of a coiled spring 3Bp, is provided with the bearing structure. Accordingly, unless the pressing force by the coiled spring is changed, the braking-torque (b.multidot.k) on the bearing side is substantially constant, where b is produced by multiplying the pressing force by a friction coefficient .mu., and K is the distance from the rotation center of the rotation part to the point of action of frictional force.
Thus, in view of the fact that T.multidot.R=b.multidot.k, or the braking-torque (b.multidot.k) on the bearing side maintains a close equivalence to the rotating-torque (R.multidot.T) on the roving bobbin side, the roving withdrawing-tension T exerted on the rotating part, as roving is withdrawn, increases in inverse proportion to a decrease in the outer radius R of a roving bobbin due to withdrawing of roving. This results in the so-called "two dimensional relation" between T and R. Practically, with roving bobbins now available, the radius R.sub.f of a full bobbin is 3 to 6 times larger than the radius R.sub.o of the empty bobbin. Accordingly, even if the roving withdrawing-tension T.sub.f with a full bobbin (radius R.sub.f) is set at 2.0 g., the roving withdrawing-tension T.sub.o present with an empty bobbin (radius R.sub.o) actually amounts up to 6 to 12 g., which creates a problem of going far out of the optimum allowance (3.0.+-.1 g.) according to spinning technology.
As shown in FIG. 7, in the conventional bobbin holder the space 4B in the periphery of the bearing portion is reduced by being substantially filled with the bearing structure. A fly created by dust being continually dragged into the space increases in size and finally leads to a fatal phenomenon such that it becomes tightly wound around a pivot member 3Br. Further, reciprocating shock loads produced by the doffing and donning operation of a bobbin causes steel balls 5B to hit and damage the upper and lower raceways 22R and 35R, and the impact damage progresses every time the doffing and donning operation of a bobbin is carried out. Thus, the factors accounting for a fatal defect acts synergetically, and a braking-torque is progressively increased from the value initially set, resulting in a marked and continual rise in the braking-torque (b.multidot.k) on the bearing side in an unexpectedly short period (1 to 2 years). This leads to an extreme reduction in effective life of the bobbin holder BH and to a sharp increase in problems which are fatal to both the quality of yarn and the production operation. The prior art has tried but found it difficult to solve these problems, so that it is now impossible to automatize production steps and make them continuous in order to obtain finer and higher-grade yarn.